This review critically summarizes the effects of photo-oxidation on the physical, chemical, and biological behaviors of micro- and nanoplastics (MNPs) in aquatic and terrestrial environments. Photo-oxidation, driven by sunlight, alters the surface properties of MNPs, influencing their adsorption of contaminants, stability, mobility, and transport of pollutants such as organic pollutants (OPs) and heavy metals (HMs). The review highlights the photochemical processes involving MNPs with coexisting constituents, emphasizing factors affecting photo-oxidation and the contribution of MNPs to the phototransformation of other contaminants. Aged MNPs exhibit distinct biological effects, including toxicity to aquatic organisms, biofilm formation, planktonic microbial growth, and microbial community changes in soil and sediment. Research gaps and future perspectives are discussed, focusing on the interaction mechanisms of MNPs with natural constituents and pollutants under photo-oxidation conditions, the combined effects of photo-oxidation and natural constituents on MNPs' fate, and the microbiological effects of photo-aged MNPs, particularly their biotransformation of pollutants. Photo-oxidation of MNPs is a complex process involving free radicals, leading to changes in physicochemical properties and inducing fragmentation and leaching of organic matter. The presence of environmental constituents such as anions, cations, and minerals can accelerate or inhibit photo-oxidation by promoting or scavenging reactive oxygen species (ROS). Surface property modifications after photo-oxidation, such as increased O-containing functional groups and decreased hydrophobicity, can change the fate of MNPs in the environment. Photo-oxidation can enhance or reduce the adsorption capacity of MNPs toward pollutants, affecting their mobility. Additionally, photo-oxidation can produce environmentally persistent free radicals (EPFRs) and ROS, mediating the photochemical transformation of pollutants such as OPs, HMs, and engineered nanoparticles (ENPs). The physical behavior of MNPs after photo-oxidation includes changes in adsorption of contaminants, colloidal stability in water media, and transport behavior in porous media. Photo-oxidation can influence the adsorption of OPs and HMs by altering surface properties and interaction mechanisms. Colloidal stability in water media is affected by the presence of monovalent and divalent cations, natural colloids, and minerals. Photo-oxidation can enhance or reduce the stability of MNPs depending on the environmental conditions. Transport behavior in porous media is influenced by changes in surface properties and interactions with soil media. Photo-oxidation can increase the mobility of MNPs in porous media, affecting the transport of contaminants. The photochemical processes with coexisting constituents involve the generation of reactive species and interactions between MNPs and environmental components. Inorganic ions, natural minerals, and dissolved organic matter (DOM) can influence the photo-oxidation of MNPs. DOM, with its chromophores, acts as a photosensitizer, affecting theThis review critically summarizes the effects of photo-oxidation on the physical, chemical, and biological behaviors of micro- and nanoplastics (MNPs) in aquatic and terrestrial environments. Photo-oxidation, driven by sunlight, alters the surface properties of MNPs, influencing their adsorption of contaminants, stability, mobility, and transport of pollutants such as organic pollutants (OPs) and heavy metals (HMs). The review highlights the photochemical processes involving MNPs with coexisting constituents, emphasizing factors affecting photo-oxidation and the contribution of MNPs to the phototransformation of other contaminants. Aged MNPs exhibit distinct biological effects, including toxicity to aquatic organisms, biofilm formation, planktonic microbial growth, and microbial community changes in soil and sediment. Research gaps and future perspectives are discussed, focusing on the interaction mechanisms of MNPs with natural constituents and pollutants under photo-oxidation conditions, the combined effects of photo-oxidation and natural constituents on MNPs' fate, and the microbiological effects of photo-aged MNPs, particularly their biotransformation of pollutants. Photo-oxidation of MNPs is a complex process involving free radicals, leading to changes in physicochemical properties and inducing fragmentation and leaching of organic matter. The presence of environmental constituents such as anions, cations, and minerals can accelerate or inhibit photo-oxidation by promoting or scavenging reactive oxygen species (ROS). Surface property modifications after photo-oxidation, such as increased O-containing functional groups and decreased hydrophobicity, can change the fate of MNPs in the environment. Photo-oxidation can enhance or reduce the adsorption capacity of MNPs toward pollutants, affecting their mobility. Additionally, photo-oxidation can produce environmentally persistent free radicals (EPFRs) and ROS, mediating the photochemical transformation of pollutants such as OPs, HMs, and engineered nanoparticles (ENPs). The physical behavior of MNPs after photo-oxidation includes changes in adsorption of contaminants, colloidal stability in water media, and transport behavior in porous media. Photo-oxidation can influence the adsorption of OPs and HMs by altering surface properties and interaction mechanisms. Colloidal stability in water media is affected by the presence of monovalent and divalent cations, natural colloids, and minerals. Photo-oxidation can enhance or reduce the stability of MNPs depending on the environmental conditions. Transport behavior in porous media is influenced by changes in surface properties and interactions with soil media. Photo-oxidation can increase the mobility of MNPs in porous media, affecting the transport of contaminants. The photochemical processes with coexisting constituents involve the generation of reactive species and interactions between MNPs and environmental components. Inorganic ions, natural minerals, and dissolved organic matter (DOM) can influence the photo-oxidation of MNPs. DOM, with its chromophores, acts as a photosensitizer, affecting the